The server_script_url is one of the back-end jPFChat settings under the "Advanced" tab. It's there to help with some very specific SEF issues that seem to come up from time to time.
If you're using SEF and are having trouble getting jPFChat to function, it's probably because the SEF is interfering with jPFChat's ability to find its own script. That's where the server_script_url setting comes in.
So... just look at the URL when you click the menu link to get to the chat page. Cut and paste it *exactly* and *entirely* into the server_script_url setting box. That's it. It will either work for you or not. If it doesn't... then then problem is elsewhere, and you should leave this setting blank.